The Moon looks fixed in the sky, but it is slowly leaving us. Each year it drifts roughly 3.8 centimetres farther from Earth — about the speed your fingernails grow. Over millions of years that adds up, and it is changing something surprising: the length of our day.
How we know it is happening
This is not a guess. The Apollo astronauts and several robotic missions left retroreflectors on the lunar surface — mirrors that bounce laser beams sent from Earth. By timing how long the light takes to return, scientists measure the Earth–Moon distance to within a few centimetres. The verdict is clear and consistent: the gap is widening.
Blame the tides
The Moon’s gravity raises tidal bulges in Earth’s oceans. Because our planet spins faster than the Moon orbits, those bulges are dragged slightly ahead of the Moon. Their gravity tugs the Moon forward, nudging it into a higher orbit — while friction from the tides gradually slows Earth’s spin.
Our days are getting longer
As Earth’s rotation slows, days stretch out. Hundreds of millions of years ago a day lasted only about 22 hours. The change is tiny on human timescales — milliseconds over a lifetime — but it never stops.
What happens in the far future
Billions of years from now the Moon will sit much farther away, total solar eclipses will no longer be possible, and Earth’s day will be far longer than 24 hours. For now, though, the Moon is simply taking its slow, steady leave — one fingernail’s width at a time.
Photo: NASA / public domain.